The present invention relates to power plants and pertains particularly to multi-boiler co-generating plants with a steam turbine generator and process steam requirements with flexible control of reheat steam and process steam supply.
Many manufacturing and processing plants require a source of steam to carry out many of the processes. For this reason most factories and processing plants have their own steam generating plants. For economic reasons many of the steam generating plants are designed and built as co-generating plants supplying steam for processing and for powering turbines for generating electrical power.
Co-generation power plants wherein boilers supply steam for processing as well as to turbines for generation of electrical power are widely used in industry today. It is common for such systems to be called on to supply widely varying demands for process steam and turbine power steam. Many times the temperatures and pressures for the process steam and the turbine generator steam are different. In some cases, when high volumes of process steam is required, boilers are designed for supplying process steam only. However, where the power plant has multiple boilers supplying the steam to the turbine generator, a continuous uninterruptable supply of process steam can normally be maintained.
Many factors enter into the economical construction and operation of a power plant. For example, fuel efficiency increases with higher pressures and temperatures. However, plant construction and operating costs generally increase rapidly with increases in pressures and/or temperatures. Higher pressures and temperatures require expensive heavier and higher strength materials.
Where process steam requirements are high, and when the steam is off-line, reheater protection under high boiler firing conditions becomes a major problem. However, this can sometimes be remedied with a high pressure (HP) turbine bypass system. Also where the process steam pressure requirement is less than the reheat system pressure, the main steam bypassed through the reheater can meet the process steam requirement.
If the process steam supply pressure is greater than the reheater design pressure, process steam can be supplied from the main steam line. Then, reheater protection becomes a problem because there in no cooling flow through the reheater.
Accordingly there is a need for an arrangement and interconnection of the boilers, turbines, reheaters and process steam supply that enables flexible operation and protects the reheater where different pressures are required.